PSN-L Email List Message

Subject: Re: OpAmp noise
From: "David Saum" DSaum@............
Date: Sun, 6 Oct 2013 11:18:49 -0400 (EDT)

> From:    Brett Nordgren 
> Date:    Sat, 05 Oct 2013 14:51:37 -0400
> Chris
> You wrote:
>>Subject: Re: nice online Opamp Input Noise Voltage Tutorial
>> >What is the noise at DC ?
>>Actually, DC by definition, does not vary, and so has zero noise.
>>****Umm ? DC = Direct Current. The Voltage is NOT defined !
> I agree.  An ideal DC source doesn't have zero noise, it's just that
> noise density simply has no meaning at DC.
>>There's also the possibility of using an auto-zero (chopper) device.
>>They work well with both high and low impedance sensors, but
>>they are at their best at low frequencies, below a few Hz.
>>****These are VERY NOISY compared to TRUE chopper amplifiers.

for a very low noise chopper opamp with NO 1/F noise, see the ADA4528

> We needed to keep our circuits small and simple and so took the
> approach that if something couldn't demonstrate that it improved
> performance we didn't use it.  In your reference, for one of the
> chopper circuits I noticed the comment "In general, to maintain low
> noise performance, source resistance should be kept below 500
> Ohms."  so I'm not exactly sure what that implies for our circuits.
>> >There has to be one best opamp for 100Hz to 100seconds,
>> >What Op Amp is it ??
>>Unfortunately, the choice will depend entirely on the circuit it is
>>being used in.  In the FBV designs, after much testing, the AD706 has
>>proven to be a truly excellent all-around device.
>>****Also look at the OPA227 and the LT1007CN?
> Per your suggestion I looked at those devices vs the AD706.  I
> measured the "typical" noise curves of all three to get the data below.
> Here the noise is expressed as a constant noise density at higher
> frequencies and a corner frequency, below which the noise becomes 1/F
> in nature.
> Voltage noise density, nV/root Hz     Corner frequency, Hz
> AD706         14.6          8.45Hz
> OPA227       2.47         19.9Hz
> LT1007CN   3.14         36.9Hz

ADA4528       ~6          0Hz (no 1/F noise)

Since we are often interested in teleseismic signals <0.1Hz
the chopper ADA4528 with no 1/F is much superior at these low frequencies.

> Current noise densiity - fA/root Hz,    Corner frequency - Hz
> AD706           44.6        4Hz
> OPA227         379        26.7Hz
> LT1007CN     434        36.9Hz

ADA4528        500        0Hz (no 1/F noise)

The ADA4528 looks highest noise here at first glance
but the steep increase of 1/F noise below their 1/F corner frequencies
for the others negates this again.

> As mentioned, it all depends on the circuit, and in particular the
> resistance, R, of the source circuit being sensed by the IC.  The
> AD706 has significantly lower current noise and lower 1/F corners and
> so should perform best with higher R values and at lower frequencies.
> For R > 15k  The AD706 is the best from DC up to 1Hz

See this app note on the ADA4528 for noise calcs vs source resistance

My testing has not found any better opamp for telesesimic frequencies and
source resistances from 100 ohms to 10k ohms (geophones to coil/magnets).



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